Field of the Invention
The present invention generally relates to prism units and projectors, and more particularly, to a prism unit that combines light in the three colors of R (red), G (green), and B (blue) on the same optical axis, for example, and a three-plate projector including the prism unit.
Description of the Related Art
Digital micromirror devices are known as reflective display devices mounted on projectors. A digital micromirror device has an image display surface formed with minute micromirrors, and controls the tilt of each mirror surface on the image display surface, to modulate the intensity of illumination light and thus form an image. Switching on and off of each pixel of the digital micromirror device is expressed by ±12-degree rotation of the mirror surfaces about a rotational axis at an angle of 45 degrees with respect to each side of the image display surface (or micromirror drive about one axis), for example. As for micromirror movement, a digital micromirror device of a new operation type (Tilt & Roll Pixel DMD) that drives micromirrors with respect to two axes perpendicular to each other is suggested in DLP Tilt & Roll Pixel Architecture and DLP IntelliBright™, <URL:http://www.dlp.com/pico-projector/pico-product-developers/2trp-chip.aspx>.
In a projector that uses a reflective display device such as a digital micromirror device and a color separating/combining prism, the angles of incidence with respect to the dichroic coatings in the color separating/combining prism differ between illumination light and projection light (on-state light). Therefore, light loss is caused due to a difference in angular characteristics between the dichroic coatings. So as to reduce such light loss, JP 10-104763 A, JP 10-319344 A, and JP 11-142992 A suggest projectors in which the angles of light incidence with respect to dichroic coatings are adjusted.
In each of the projectors suggested in JP 10-104763 A, JP 10-319344 A, and JP 11-142992 A, a prism unit that first separates G from the three colors of R, G, and B is provided, and the separation cutoff between B and R is set almost at the center of the reflection band of G. A prism unit that first separates G is known as a prism unit for video optical systems, LCOS (reflective crystal liquid) projectors, and the like. In that color combination, only projection light should be taken into consideration, and therefore, the separation wavelength between B and R does not affect performance at any part of the G wavelength band.
For example, in a projector optical system compatible with digital micromirror devices of the above described new operation type, the angle of incidence at the time of separation/combination of B and R differs between the illumination light path and the projection light path. The cutoff wavelength varies with the angle of incidence in a dichroic coating. Therefore, if the rising wavelength position is set by using a BR dichroic coating having a film structure that takes into account only the projection light path, there is a possibility that efficiency will rapidly drop in the illumination light path. Therefore, with the angular characteristics of the dichroic coatings disclosed in JP 10-104763 A, JP 10-319344 A, and JP 11-142992 A, it is difficult to sufficiently reduce light loss in illumination light.